This application claims priority under 35 USC xc2xa7119 to Japanese patent application JPAP10-190811 filed Jul. 6, 1998, the entire contents of which is incorporated herein by reference.
1. Field of the Invention
The present invention generally relates to a method and apparatus for image processing, and more particularly to a method and apparatus for image processing which efficiently performs a gamma conversion operation using a digital signal processor (DSP).
2. Discussion of the Background
An image processing apparatus such as a copier machine scans a document sheet placed on a contact glass thereof and performs opto-electrical conversion on each scanned line. The obtained image signal is converted into a digital signal, which is then subjected to image processing to generate a recording signal. The image processing apparatus uses this recording signal to modulate laser light emitted from a laser device, thereby forming a latent image on a photo-sensitive body. This latent image corresponds to the original document image.
The image processing performed during the above operation includes such processes as color correction, gamma correction, shading correction, MTF correction, gray-scale correction, size enlargement/reduction process, etc.
Conventional image processing apparatuses conduct gamma correction by using a look-up table stored in a memory. Namely, a look-up table, which lists output levels corresponding to input levels over the entire gray-scale range, is stored in an external memory, and an input address representing an input level is supplied to read a corresponding output level from the memory. An input-to-output relation between the input levels and the output levels can be drawn as a characteristic curve on a graph, which is generally referred to as a gamma curve.
The memory may need to store more than one look-up table, i.e., more than one relation between input levels and the output levels. Such a plurality of gamma curves may become necessary when the copier machine is provided with a function to select a desired image density from a plurality of image density levels for printing images, for example. This adds to memory space required for storing the look-up tables.
In some configurations where an image memory is shared by different apparatuses, separate gamma corrections may become necessary for a scanner and for a printer, for example. This also adds to the memory space for the look-up tables. Also, when there is a need to change gamma curves, all the entries in the look-up tables have to be rewritten.
In this manner, the scheme which stores gamma curves as look-up tables requires a relatively large memory space to store a large data set and lacks flexibility in changing gamma curves.
Accordingly, there is a need for an image processing apparatus which can reduce the size of the stored data set required for gamma correction and allows the gamma curves to be easily modified.
Accordingly, one object of the present invention is to provide a novel image processing apparatus which efficiently performs a gamma conversion operation.
Another object is to provide a novel method of image processing which efficiently performs a gamma conversion operation.
To achieve these and other objects, the present invention provides a novel image processing apparatus which includes a memory, a plurality of parameter registers, and a gamma conversion processor. The memory has stored therein at least one gamma conversion algorithm and a plurality of parameter sets corresponding to predefined gray-scale ranges. The gamma conversion processor selects a suitable parameter set from among the plurality of parameter sets stored in the memory according to a required output gray-scale range, and transfers the selected suitable parameter set from the memory to the parameter registers sequentially during a gamma conversion operation. Then, the gamma conversion processor performs at a time in parallel the gamma conversion operation with respect to input gray-scale values of successive pixels of an input image data stream using the at least one gamma conversion algorithm stored in the memory and the selected suitable parameter set sequentially stored in the parameter registers, and generates a serial output gray-scale signal based on a result of the gamma conversion operation.
The image processing apparatus may includes a gray-scale range controller which determines the required output gray-scale range.
The present inventions provides another novel image processing apparatus includes a re-programmable memory, a gray-scale range controller, a plurality of parameter registers, a plurality of calculators, and a gamma conversion processor. The re-programmable memory previously stores a gamma conversion algorithm and a plurality of parameter set groups corresponding to a plurality of predefined gray-scale ranges. Each of the parameter set groups includes a plurality of parameter sets corresponding to a plurality of predefined gray-scale segment numbers. The gray-scale range controller determines a required output gray-scale range. The plurality of calculators are provided corresponding to a plurality of successive pixels in a raster of an input image data stream. Each of the calculators performs a gamma conversion operation with respect to one pixel. The gamma conversion processor determines a gray-scale segment number according to an input gray-scale range and divides the input gray-scale range accordingly into the determined gray-scale segment number. The gamma conversion processor selects a suitable parameter set from among the plurality of parameter sets stored in the re-programmable memory according to the required output gray-scale range determined by the gray-scale range controller and the determined gray-scale segment number and sequentially transfers the selected suitable parameter set from the re-programmable memory to the parameter registers during a gamma conversion operation. Then, the gamma conversion processor causes the plurality of calculators to perform at a time in parallel the gamma conversion operation with respect to the input gray-scale ranges of successive pixels of an input image data stream using the gamma conversion algorithm and the selected suitable parameter set stored in the parameter registers, and generates a serial output gray-scale signal based on a result of the gamma conversion operation.
Each of the plurality of parameter sets may include a predefined gray-scale segment number and combinations of values of x- and y-coordinates and a value of a slope for respective gray-scale segments in a corresponding gray-scale range.
The re-programmable memory may include a plurality of different gray-scale segment numbers corresponding to a plurality of different gray-scale ranges, and the gamma conversion processor may determine the gray-scale segment number by selecting one of the plurality of different gray-scale segment numbers according to the required output gray-scale range determined by the gray-scale range controller.
The re-programmable memory may be connected to an external computer and previously store a gamma conversion algorithm and one parameter set corresponding to one predefined gray-scale range, and a predefined gray-scale segment number, the one parameter set including a plurality of parameters corresponding to a predefined gray-scale segment number. The apparatus may further includes a selector for selecting one of a connection of the re-programmable memory within the apparatus and a connection between the re-programmable memory and the external computer. In this case, the external computer causes the selector to select the connection between the re-programmable memory and the external computer, and changes contents of the one parameter set and the predefined gray-scale segment number in response to a required output gray-scale range determined by the gray-scale range controller.
The image processing apparatus may further includes a display and data entry unit which is connected to the external computer and the re-programmable memory. In this case, the contents of the one parameter set and the predefined gray-scale segment number are changed through the display and data entry unit.
The display and data entry unit may read the contents of the one parameter set and the predefined gray-scale segment number from the re-programmable memory and indicate the read contents thereon.
Also, the present invention provides a method of image processing which includes the steps of storing, establishing, providing, determining, dividing, selecting, transferring, causing, and generating. The storing step previously stores a gamma conversion algorithm and a plurality of parameter set groups corresponding to a plurality of predefined gray-scale ranges. Each of the parameter set groups includes a plurality of parameter sets corresponding to a plurality of predefined gray-scale segment numbers into a re-programmable memory. The establishing step establishes a required output gray-scale range. The providing step provides a plurality of calculator corresponding to a plurality of successive pixels in a raster of an input image data stream. Each of the calculators capable of performing a gamma conversion operation with respect to one pixel. The determining step determines a gray-scale segment number according to an input gray-scale range. The dividing step divides the input gray-scale range accordingly into the determined gray-scale segment number. The selecting step selects a suitable parameter set from among the plurality of parameter sets stored in the re-programmable memory according to the required output gray-scale range established in the established step and the determined gray-scale segment number. The transferring step sequentially transfers the selected suitable parameter set from the re-programmable memory to a plurality of parameter registers during a gamma conversion operation. The causing step causes the plurality of calculators to perform at a time in parallel the gamma conversion operation with respect to the input gray-scale ranges of successive pixels of an input image data stream using the gamma conversion algorithm and the selected suitable parameter set stored in the parameter registers. The generating step generates a serial output gray-scale signal based on a result of the gamma conversion operation.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings.